1. Field of the Invention
The present invention relates to a second-harmonic choking filter employed in a strip type microwave transmission line.
2. Description of the Related Art
In a microwave radio transmission apparatus, there is employed a frequency converter which includes a local frequency oscillator outputting a local frequency f.sub.LO and a non-linear element, such as a diode or a transistor, so as to convert an input signal having frequency f.sub.s to a signal having a frequency (f.sub.LO -f.sub.s) or (f.sub.LO -f.sub.s). At this time, unnecessary signals, spurious emissions, having frequencies 2f.sub.LO, 3f.sub.LO . . . are also output. Among these frequencies, the second harmonic wave 2f.sub.LO of the local oscillator is of the highest level, and sometimes becomes even higher than the level of the necessary frequency-converted signal. Therefore, a second-harmonic choking filter provided therein must fully choke, i.e. prevents, the second-harmonic wave to propagate, while the performance of the necessary signal is not deteriorated even installed in a limited space and its adjustment must be easy.
FIG. 1 shows a prior art structure of a second-harmonic wave choking filter formed with a strip-type transmission line; and FIG. 2 shows an admittance Smith Chart for explaining the operation of FIG. 1 filter circuit. From the left hand side into FIG. 1 filter a fundamental frequency wave to be transmitted therethrough and its second harmonic wave to be choked thereby are simultaneously input. As shown in FIG. 1, a main transmission line 2 constituted of a strip-type transmission line is provided with open stubs 1 and 3, each constituted of the same strip-type transmission line as the main transmission line 2, having the longitudinal length of Lg/8, and each separated by a distance L along the main transmission line 2, where Lg indicates an effective wavelength of the fundamental frequency wave on the transmission lines 1, 2 and 3. Accordingly, these open stubs 1 and 2 have effectively a quarter wave length for the second-harmonic frequency wave. When the open stubs 1 and 3 are connected to an arbitrary position A on the main transmission line 2, the admittance looking at the right hand side of the main transmission line 2 is the characteristic admittance Y.sub.O of the main transmission-line because of no reflection, therefore, falls on the center of the admittance Smith Chart of FIG. 2. The open stub 1 having the wave length Lg/8 connected to the position A shifts the above-described admittance from the center to an admittance denoted with A.sub.1 in FIG. 2. Therefore, a part of the fundamental wave on the main transmission line 2 is reflected, and the rest is transmitted towards the output side, i.e. the right hand side of the main transmission line. At this state, the second-harmonic wave is fully reflected at position A because the open stub 1 having a quarter wavelength of the second-harmonics wave looked at from position A exhibits an infinite admittance, i.e. equivalent to a shorted state. At a position B which is advanced on the main transmission line by a distance L from position A, if the second open stub 3 is not connected to the main transmission line 2 yet, the admittance becomes that denoted with the point A.sub.2, which is the conjugate of point A.sub.1, on FIG. 2. Then, by connecting the second stub 3 having the same length, i.e. same admittance as that of the first stub 1, to position B the admittance A.sub.2 is canceled so as to move back to the center. In other explanation, a of the fundamental frequency wave is reflected also at position B; however, the reflected wave at position B cancels the reflected wave at position A. Thus, the transmission line 2 allows the fundamental wave to propagate to the right hand side without reflection.
When the distance L between the two stubs 1 and 3 is varied, the impedance moves along the most central coaxial circle C.sub.1 of FIG. 2. When the length of the stub connected to position B is varied, it moves on the left hand side circle C.sub.2.
In the FIG. 1 structure, when the frequency of the fundamental wave is determined, the lengths of the open stubs 1 and 3 and the distance therebetween are uniquely determined. However, considerable area of the printed circuit board is required for installing the stubs. When the available space is limited, the main transmission line 2 must be bent, causing a deterioration of the characteristic impedance. When the actual performance is different from the designed target performance, the stub lengths and the distance L therebetween must be adjusted. Thus, there is a problem in that the limited space may deteriorate the characteristics as well as require complicated adjustments.